Lots of good points made in this thread.
Don't forget about aircraft weight and how it affects taxi maneuverability.
Departing aircraft are heavy and take a longer time to accelerate from a stop. If multiple departures are waiting to cross the inner 'arrivals' runway, it will take awhile for each of them to build momentum to cross the runway. An additional consideration is the flight attendants: during taxi-out they are up doing safety demonstrations and walking around.
On the other hand, arriving aircraft are lighter. Multiple arriving aircraft can cross the departure runway quickly because they can accelerate and maneuver more easily. Furthermore, the flight attendants and passengers are all seated and belted, so smooth taxiing is less of a consideration.